This invention relates to nuclear reactor fuel assemblies and in particular to holddown devices for such assemblies.
In conventional pressurized water nuclear reactors, each nuclear fuel assembly rests on the core support stand and is properly positioned thereon when the fuel assembly alignment posts are firmly in place against the fuel assembly alignment pins in the core support stand. During core operation coolant is pumped into the region below the core support stand, continues upward through the fuel assembly lower end fitting, then flows into the active fuel region of the core. The rapid upward flow of the coolant exerts large forces over the entire length of the fuel assembly, tending to lift the assembly from the support stand. The assembly must be held firmly against these forces, but cannot be permanently attached to the support stand because periodic refueling of the reactor requires removal or relocation of each assembly.
Prior methods of holding down the fuel assembly include spring devices at the top or bottom of the assembly which operate to bias the assembly against a fixed structure in the reactor vessel such as the fuel alignment plate at the upper end of the core or the core support stand at the bottom. Other devices have been proposed which lock the end fitting against the adjacent alignment pins as the fuel assembly is inserted into place. These kinds of devices have several disadvantages. Some, such as the vertical spring bias, exert compressive loads on structural members of the fuel assembly and contribute to bowing of the assembly after extended use. In others the holddown force is not strong enough to prevent fuel assembly vibration. Yet another problem, particularly with the devices having springs on the lower end fitting for engagement with the alignment pins, is that the force required to disengage the assembly is very high and produces undesirable wear on the alignment pins. Also, bottom mounted holddown devices are often relatively large and result in an undesirably high pressure drop between the core inlet and the fuel region of the reactor. Finally, most of these prior art devices are expensive to manufacture.